In the prior art, a vertical wall such as a door is usually provided with a peephole, or otherwise, with a small lens through a hole in the wall through which a visitor or visitors can be confirmed. Alternatively, the door is restrained from being fully opened by means of a chain device while it remains slightly open. However, such prior art devices have disadvantageously imparted discomfort to the visitor or visitors. The peephole or the small lens has a disadvantage in that one cannot communicate with the visitor or visitors in a satisfactory manner (although it does prevent crimes in a safe manner), and the chain device cannot effectively prevent crimes because any slight opening will allow an invader to enter through the door broken by force and arms.